The invention relates to the manufacturing of iron and steel and more particularly, to a process for direct iron and steelmaking.
My U.S. Pat. No. 5,542,963 describes a process for direct iron and steelmaking featuring the combination of solid-state iron oxide reduction by pressurized hot reducing gases followed by continuous melting of the hot reduced iron. This invention comprises a modified process extended to be more versatile, including several improvements and additions. This application is a continuation-in-part of co-pending application Ser. No. 08/916395.
There are a large number of known processes and variations thereof for accomplishing solid-state iron oxide reduction, all of which achieve the object of producing direct reduced iron, known as DRI or sponge iron, as the end product. These may conveniently be divided into the following two groups:
Group A: Solid-state reduction processes employing pressurized hot reducing gases percolated through a gravity contact-supported or fluidized columnar moving bed of iron oxide particles, pellets or lumps, wherein the pressurized reducing gases comprise recirculated top gases enriched by externally reformed hydrocarbons and/or directly introduced hydrocarbons and the in-bed pressure of the reducing gases at product discharge is substantial, typically in the range of 1-5 atmospheres; and
Group B: Solid-state reduction processes employing coal or other solid carbonaceous reductant, either mixed with iron oxide pellets or lumps as discrete particles, or as a constituent of agglomerated (pelletized) iron oxides, traversing along elongated and relatively shallow moving beds within rotary kilns or carried upon rotary or traveling hearths, and which include a non-recirculated heating gas phase over the bed, as well as reducing gas phase within the bed generated by in-bed reaction of the coal, and wherein the process gas pressure at product discharge is close to zero relative to ambient atmospheric pressure.
Group A processes for DRI production include, for example, MIDREX, HYL, PUROFER, NIPPON STEEL-DR, and AREX-SBD featuring gravity contact-supported descending moving beds within shaft furnace reactors and the FIOR, FINMET, SPIREX, CIRCORED and CIRCOFER processes in fluidized beds and the iron carbide processes, either with fluidized or gravity contact-supported beds.
Group B processes include SL/RN, DRC, KRUPP-CODIR and ACCAR (with coal) processes employing discrete particle iron-coal mixtures heated within rotary kilns. The FASTMET and INMETCO processes employ pelletized mixtures of fine particulate iron oxide and coal, and COMET alternate layers or iron oxides and a coal/limestone mixture, heated upon rotary hearths.
An overall object of the present invention is to combine in-process features from known processes in these two groupings within the art of solid-state iron ore reduction together with continuous metal melting, comprising a continuous sequence of process steps to produce liquid iron and steel directly from iron oxides, to realize higher output with improved control of product composition and quality, lower energy requirements, higher metal yield with lower material losses, and lower discharge of environmental pollutants than by currently known processes or combinations of processes.
When considering the solid-state reduction process stage, it is not notable that the ACCAR process, when operated with only natural gas or fuel oil, is an exception outside the two above groupings, because it operates near atmospheric pressure without solid carbonaceous reductant. Between 1967 and 1977, in pilot and demonstration rotary kiln plants operated at near-atmospheric pressures, it was shown that hydrocarbons in the form of natural gas or oil, when injected directly into a hot bed of iron oxide pellets, were reformed into reducing gases (CO+H2) within the bed itself, obviating the need for external reforming. This work was summarized in xe2x80x9cDirect Reduced Iron-Technology and Economics of Production and Usexe2x80x9d, Iron and Steel Society, AIME, 1980, pp. 87-90. Only after about ten or more years was this conceptual breakthrough also applied to Group A processes, for example, AREX technology as an alternative to other shaft furnace technologies which use externally reformed gases for reduction. A wide range of reducing gas makeups are therefore workable, appearing necessary only that approximately suitable temperatures and ratios between H, C and O be maintained in the reducing gas, for sustainable solid-state reduction to metallic iron to proceed. This latitude allows the selection of features for solid-state iron oxide reduction circuits to be more freely focused upon such objects as low process energy requirements, high production rate, low volumes of waste gases containing less particulates and unburned combustibles, improved control of product composition, simplicity and low costs.
The various oxygen converter and electricxe2x80x94arc furnace processes dominate current commercial steelmaking practice, but share a common problem of unburned combustibles CO and H2 contained in the off-gases. The known bath smelting processes also share this difficulty. The development of post-combustion technology has mitigated this problem, but the post-combustion degree (PCD) continues to vary widely during different stages of each heat of steel and substantial excess oxygen via multiple furnace gas-stream, injectors is a typical requisite. The heat transfer efficiency (HTE) of in-furnace utilization of the heat so-generated is also relatively low, mainly because of the batch-wise operating mode, typical EAF or BOF geometric shape, and remoteness of the bath from the gas stream exit. Subsequent utilization, such as for preheating scrap, has been only marginally viable as typically somewhat complex and costly to apply in practice. One object of the invention is to realize consistent and near-complete post-combustion including efficient in-furnace heat transfer to the charge, as characterized by uniformly high PCD and HTE and also efficient utilization within the process system of the remaining sensible heat contained in the off-gases from melting, thereby minimizing overall process energy requirements and discharging into the atmosphere only substantially combustible-free exhaust gases at low temperatures.
These current iron and steelmaking processes almost universally feature lancing or sub-surface injection of high-purity oxygen into the bath, typically at high pressures and high velocities in the sonic range. The chemical combination of some of the oxygen with iron generates iron oxide fume which is exhausted as fine particulates, with the effect of reducing metal yields and polluting the environment. Another object of the present invention is to provide a steelmaking process which does not inherently involve injecting oxygen into the bath, using it only when needed for handling specific process materials and special product requirements. A corollary object is to substantially decrease the generation of iron oxide fumes which are typical of current commercial steelmaking processes. Another corollary object is providing the application for low-pressure oxygen of lower purity, such as generated from air separation by molecular sieves, instead of high-purity, high-pressure oxygen.
Still another object of the invention is to release only a minimum volume of exhaust gases at relatively low temperature which are substantially free of combustibles, thereby carrying less heat losses and pollutants into the atmosphere than other overall iron ore reduction and steelmaking combinations.
A further object is to accomplish transfer and immersion of hot solid reduced iron from the reduction stage into a partially melted metal bath at the melting stage with minimum time, heat loss and contact with the ambient atmosphere, furnace gases and steelmaking slag cover.
A still further object is to distribute the hot solid reduced iron pieces at entry into the partially melted metal bath and disperse them sufficiently to avoid the formation of agglomerated floating islands of unmelted iron pieces and in-bath minimize slow mass transfer as a barrier to fast heat transfer and melting.
Yet another object is to employ a minimum quantity of raw materials, additives, fuels, reductants and oxidants in a direct iron and steelmaking process, in which all of the principal process steps can be conducted simultaneously and continuously to yield a continuous stream of liquid iron and steel having a controlled composition and temperature.
As applied to foregoing Group A processes, the invention provides a process for direct iron and steelmaking which comprises introducing iron oxides containing pieces into a gas-solid reduction zone within a reduction reactor fired by pressurized hot reducing gases comprising recirculated top gases enriched by externally reformed hydrocarbons and/or directly introduced hydrocarbons and percolating said reducing gases through said gas-solid reduction zone for reaction with said iron oxides yielding hot solid reduced iron pieces, followed by transferring said reduced iron pieces into a gas-solid-liquid melting zone containing a partially melted metal bath carried within the inner side walls of an elongated rotary furnace having at least a partial top cover of floating slag and fired by combustible and oxygen-containing gases generating a gas stream of hot furnace gases passing above the bath surface supplying heat for continually melting said hot reduced iron to yield liquid iron and steel, said gas stream exiting through an annular end opening of said furnace, including the following steps, in combination: advancing said hot reduced iron along with any accompanying hot reducing gases from within said gas-solid reduction zone into a transfer duct directly communicating between said reduction and melting zones and incorporating an injection lance directed through an annular end opening of said rotary furnace into said melting zone angled downwards towards said bath surface; introducing pressurized carrier gases into said transfer duct entraining and propelling said hot solid reduced iron pieces through said injection lance projecting a jet of said carrier gases and hot reduced iron pieces from said lance penetrating said metal bath surface thereby submerging and dispersing said solid reduced iron pieces within said partially melted metal bath; and dispersing said reduced iron pieces further within said metal bath following said submerging by means of the propelling action of said inner side walls rotating against the bottom perimeter of said metal bath.
Sensible heat contained in the rotary furnace hot combustion products is preferably used as part of the preheat requirement of said hot reducing gases. Externally reforming a minor portion of the enriching hydrocarbons by partial oxidation with oxygen is also a preferred feature, introducing the remaining major portion directly, as followed by in-situ reforming into reducing gases CO and H2 within the gas-solid reduction zone.
When incorporating solid-state reduction under Group B, at or near ambient atmospheric pressure, the invention includes an additional step of advancing the hot reduced iron into a pressurizing zone and applying an elevated pressure therein by introduction of a pressurizing gas. Discrete particles of carbonaceous reductant, as coal char or the like, when present mixed together with the hot reduced iron pieces, are preferably removed by size-separation prior to iron pressurizing; transfer and injection, with the char subjected to cleaning and recycling of this unreacted reductant material. The hot reducing gases emitted from the melting zone are preferably transferred into the gas-solid reduction zone providing supplementary heat for iron oxide reduction.
As applied to both A and B, the principal process steps are preferably and advantageously conducted continuously and simultaneously whilst the charge flows continually from charge to discharge. The process of the invention includes introducing additional oxygen-containing gases, such as by injecting at least 80 percent pure oxygen into the gas stream, which is directed to effectively realize (1) post-combustion of CO evolving out of the bath surface from combination between carbon and oxygen as residual iron oxides contained in the hot reduced iron, and (2) reaction with any combustibles contained in the carrier gases and accompanying hot reducing gases evolving from the jet, forming CO2 and H2O within the gas stream prior to said stream exiting the melting zone, thereby supplying additional heat for melting. In addition to the distributing and dispersing effects of the solids injection lance stream impact area in combination with furnace wall rotation and slope, a preferred feature is distributing the area of impingement of the jet of carrier gases and hot reduced iron longitudinally along the partially melted metal bath to facilitate the mass and heat transfer requirements for most efficient melting, as by traversing the jet alternately forwards and backwards within the melting zone.
The process preferably includes advancing liquid metal into a gas-liquid refining zone containing a completely melted metal bath carried within the furnace and heated by a discharge end burner supplying a portion of said combustibles and oxygen-containing gases adapted to control the temperature of said melted metal bath essentially independently of the heat requirements for melting within the melting zone, agitating, homogenizing and refining the liquid metal under the controlled agitating action of the rotating furnace side walls to yield liquid iron and steel of controlled temperature and composition. The gas stream of hot combustion products from the discharge end burner also can comprise a substantial portion of the melting heat requirements. The balance of the heat for melting is supplied by post combustion and combustion of gases evolving from the jet, which can be supplemented by a charge end burner firing directly into the melting zone.
Fluxes, alloys and carburizing agents to control and adjust the chemistry of the reactions in the gas-solid-liquid and gas-liquid reaction zones may also be introduced into said transfer duct for entrainment by said carrier gases and injection together with the hot reduced iron. Supplementary iron and steel scrap, pig iron, cold DRI or DRI briquettes may optionally be charged into the melting zone, as well as provision for longitudinally traversing the point of introduction longitudinally forwards and backwards.